In U.S. Pat. Nos. 4,781,860 and 4,775,494 there are disclosed methods for solidifying liquid wastes by incorporating sodium montmorillonite, attapulgite or sepiolite clays. The disclosure of the aforesaid patents are incorporated herein by reference. The resulting unpourable and free standing solid masses have been accepted as meeting most State, U.S. Environmental Protection Agency and U.S. Nuclear Regulatory Commission standards for disposing of such hazardous or radioactive liquid materials. For certain applications it is also desirable to achieve a relatively hard, more rigidly stabilized composition than those prepared using the aforesaid clays.
Another important requirement for the solidified compositions is resistance to leaching. Specifically, the Environmental Protection Agency specifies the requirements for leach resistance of the solidified compositions, which standards are determined by a prescribed test referred to as Toxicity Characteristic Leaching Procedure or TCLP. These standards set maximum leaching limits of various contaminants, including metals and aqueous organic compositions, from the solids containing such waste components. Another important factor is the expansion of the solidified composition; that is, the volumetric expansion of the final solidified mass as compared to the initial liquid or sludge volume. The expansion characteristics of the solidified mass are critical to the acceptability of the solidifying agent to achieve a solid mass suitable for disposal and/or storage. If expansion as a result of treatment of the waste liquid or sludge is significant, the amount of waste that can be solidified in a drum or other container or storage vessel is substantially reduced, the number of drums used for storing the same amount of waste to be solidified will be significantly increased, as will the cost of transporting, handling and storing the solidified composition. Normally, an expansion of more than about 25% of (or 1.25) times the original waste volume will be a serious economic problem, due to the high cost of land disposal.
In attempting to improve the hardness and strength of the solidified mass, the use of Portland cement has been proposed For example, in U.S. Pat. No. 4,149,968 there is disclosed a method of solidifying a hazardous liquid using a mixture of bentonite clay and Portland cement. However, when those materials are mixed and added to an aqueous liquid, or are added simultaneously to aqueous liquids as proposed by the aforesaid patent, because of the great affinity of the hydrated cement material for clay, the hydrated cement quickly reacts with or absorbs a substantial amount of the sodium and/or calcium thereby substantially reducing the effectiveness of the bentonite to capture the toxic components of the waste. Similarly, where Portland cement and sodium montmorillonite are combined and simultaneously added to aqueous liquids, the sodium is so absorbed by the hydrated cement that the clay is rendered substantially ineffective for adequate chemical stabilization of toxic components.
Attempts to resolve the aforesaid problems by sequential addition of the solidifying materials have not been generally satisfactory. For example, if Portland cement is first added to a typical Chromium waste liquid, followed by addition of the sodium montmorillonite, about 25% of the liquid will not be solidified. By reversing the sequence, although the amount of free standing or unsolidified liquid is reduced to about 12%, the results are still not generally acceptable for legal land disposal. Moreover, this sequential addition requirement precludes using pre-mixed materials, resulting in less efficient and less convenient processing of the liquids to be solidified. The present invention obviates the aforesaid disadvantages.